Filter and method of backwashing same



July 31, 1951 N. cooPERsoN Erm. 2,562,699

FILTER AND METHOD 0F BACKWASHING SAME Filed June 17, 1949 Patented July 31, 1951 l rana FILTER AND METHOD OF BACKWASHING SAME Norman Cooperson, Camden, and Albert W.v

Kingsbury, Moorestown, N. J., assignors to The Permutit Company, New York, N. Y., a corporation of Delaware Application June 17,1949,seria1N0.99,6o2

Y 3 Claims. l

This invention relates to lters of the type having toraminous means and employing as the iiltering material diatomaceous earth deposited on said foraminous means, and to an improved method of backwashing such illters.

Filters of this type have become well known and reference is made, by way of an example, to the U. S. Patent 2,423,172 of George Martin Booth, dated July 1, 1947. which discloses a filtering apparatus and a method of operating the same on which our invention is an improvement.

It is an object of our invention to provide an improved filtering apparatus which is adapted for more eiiicient operation and which can be constructed with less height than has heretofore been possible; another object is to provide a more eiective method of backwashing.

'I'he manner in which these objects are achieved is shown in the accompanying drawings in which:

Fig. 1 shows an apparatus in accordance with my invention in elevation, partly in section; and

Fig. 2 is a fractional plan view of the apparatus with the domed cover I5 removed.

Similar numerals refer to similar parts throughout the several views.

The apparatus comprises a tank I8 with a cylindrical side and a dished bottom I2 to which is attached a pocket I3.. To the top oi e the cylindrical portion H is welded a tube sheet I4. Over the tube sheet is fitted a domed cover I5 attached to the tube sheet |4 by means of a ilange I6 and bolts and nuts I1. The space within tank I and below the tube sheet |4 forms an influent chamber I8, and the space within the domed cover I and above the tube sheet I4 forms an eiiiuent chamber I9.

A pump 20 is provided with a suction pipe 2| receiving the liquid to be illtered and with a discharge pipe 22 fitted with a valve 23 and leading to the pocket I3. A branch 24 of the discharge pipe 22 is provided with a valve 25 and terminates in a bent portion 26 within the influent chamber I8 adjacent to the dished bottom I2. Leading from the lowest part of the pocket I3 is a drain pipe 28 tted with a valve 29. A funnel 3| is provided for the introduction of filter .aid and connected with the influent chamber I8 by a pipe 32 provided with a valve 33. A pipe 35 leads from a point as high as possible within the influent chamber I8 through a quick-opening valve 36 to the outside. An outlet pipe 38 leads from the level B B in the middle portion of the eiliuent chamber I9 through the cover I5, as shown, and has a branch 39 provided with a valve 40 and leading to a point of use, and another branch 4| provided with a valve 42 and leading to a point of waste. The pipe 38 also communicates with an air vent comprising a cock 43 having therein a ilow restricting passage 44. A sump 46 is arranged to receive the discharges from pipes 23 and 4| and lead them to a point of disposal through a pipe 41.

'I'he tube sheet |4 carries a number of identical lter tube assemblies each referred to as whole by the numeral 50. The lter tube assemblies 50 may employ any foraminous material capable of retaining the diatomaceous earth used as iilter aid, and we prefer to use the construction constituting the subject matter of a. co-pending application of Durando Miller, Jr. Ser. No. 99,634, iiled June 17, 1949. Each such lter tube assembly 50 comprises a support plate 5| which rests against the tube sheet I4 with an interposed gasket 52 and which is provided with openings The plate 5I has a integral sleeve 54 on which is mounted a stack of foraminous iilter elements 63 made of sintered bronze balls and rabbeted, as shown at 64, to each other, to the sleeve 54, and to a bottom plate 66 placed against the lowermost lter element 63. A chain 61 connects the bottom plate 66 with a partly threaded rod 58 passing through the plate 5I and is placed in tension by a wing nut 60. The plates 5| are held tightly against the tube sheet I4 by means of clamps 69, `studs 10 and nuts 1|.

In operating the apparatus the valves 33 and 42 are opened, all other valves being closed, and a quantity of iilter aid is introduced as a slurry into the empty tank I0 through the funnel 3| and pipe 32, and drops onto the dished bottom I2 and into the pocket I3. Valve 33 is then closed and pump 20 started, with valves 23 and 25 opened. The incoming liquid lls the tank I0, stirs up the filter aid and carries it. upward to be deposited as a layer against the lter elements 63, forming the so-called precoat. 'Ihe discharge from the bent portion 26 of pipe 24 helps to stir up all filter aid resting on the bottom I2 and to suspend it in the upflowing water. During this illing operation, air is discharged through the outlet pipe 38, valve 42,v and pipe 4I.

As the apparatus is being lled with water, air remains trapped in the iniiuent chamber I8 above the level AA and in the eiiluent chamber I9 above the level BB. As soon as the liquid has risen to the level BB the discharge of air stops and liquid ilows from pipe 4|. Such liquid is likely to be turbid at rst, containing some particles of filter aid, but such turbidity will disappear within about one minute, whereupon valve 4l is opened season and valve 42 is closed which place; the apparatus in normal filtering service. the filtered liquid iiowing via pipe 38 to a point of use. The rate of filtration may be between about 2 and 8 gallons per minute per square foot of area of the foraminous elements 68, depending on the viscosity and turbidity of the liquid to be filtered when auch liquid is water having a turbidity up to about parts per million we find a rate of about 4 gallons per minute per square foot most satisfactory. With higher rates of flow the quantity of liquid that can be filtered between backwashings becomes less.

As filtration progresses, the turbidity in the liquid is retained by the blanket of lter aid deposited on the filter elements 88 causing the pressure loss through the filter elements 83 to increase gradually, and when the pressure in the influent chamber i8 has reached a predetermined value of somewhere between 10 and 100, say 50 pounds per square inch, the normal service operation is terminated in order to backwash the filter. During filtration the pressure in influent chamber IB increases so that the air trapped in the influent chamber I8 is gradually dissolved when the liquid being filtered is capable of dissolving air, as is Water or an aqueous solution. Such air thus may disappear either completely or at least to a substantial degree, the extent depending on the existing pressure, the temperature of the liquid, and its ability to dissolve air.

To terminate the service operation the valves 4I and 23 are closed and valves 29 and 42 are opened. The apparatus now drains through the pipe 28, air entering through the pipe 4|. During this draining operation. the Jet of liquid coming in through pipe 24 helps to stir up the contents of the lower portion of the iniiuent chamber I8 and flush out all dirt and lter aid dropping from the illter elements 632 As soon as the tank has drained substantially empty valves 28 and 42 are closed and valve 28 is opened. Liquid now enters the tank I0, compressing the air as the tank is being filled. As soon as the air within the tank has been compressed to the maximum pressure which the pump 20 can produce, for instance pounds per square inch, the liquid level in tank Il will be somewhat below level AA. Now the cock 43 is opened so as to discharge air from the eluent chamber i8 through the flow restricting passage 44 without permitting the pressure within tank III to decrease appreciably. As compressed air is thus being discharged, liquid enters until it reaches the level AA, trapping a definite compressed air volume in the 'influent chamber i8 above the level` AA. Thereupon, the liquid rises through the illter tube assemblies 58 into the eiiiuent chamber I9 until it reaches the lower end of the outlet pipe 38, that is level BB. Then liquid will be discharged through the outlet pipe 28 and the cock 43, a second definite volume of compressed air being thus retained within tank i Il in the emuent chamber il above the level BB. The flow of liquid from cock 43 indicates that the tank has been prepared for the backwashing operation and cock 43 is now closed.

Valves 28 and 25 are then closed and the pump 2 8 may be shut down if desired. The quick-opening valve 28 is next opened, permitting the compressed air in influent chamber il to be exhausted rapidly to atmosphere through pipe Ii. The compressed air in eiiiuent chamber I9 consequently expands rapidly and forces the liquid in the emuent chamber I8 through the filter elemente 8l at a high rate of flow. cleansing them o! any remaining filter aid or dirt. This backwash operation takes but a fraction of a second and its end is indicated by the emergence of a small quantity of liquid from the pipe 2l. The valve 88 is now closed.

The amount of impurities dislodged during the backwash operation is relatively small. Nevertheless, it is desirable to remove them and this is accomplished by opening valves 28 and 42 to drain the tank i0 preparatory to introducing a new batch of filter aid into the funnel 8i and resuming the cycle of operations as described. During this draining operation, the pump 20 need not but may be operated with valve 28 open to more effectively cleanse the lower portion of tank Il.

The dow restricting passage 44 must be small enough to exhaust the excess of air from the eiiiuent chamber i8 without permitting the pressure therein to drop appreciably. Yet, the passage 44 should not be so small that an unnecessarily long period is required to exhaust the excess of air and leave in the tank I0 the two predetermined volumes of compressed air. The best size is one that will exhaust air at its elevated pressure at the same rate of flow at which'the pump is capable of introducing liquid against a head equal to said elevated pressure to take the place of the exhausted air. In an apparatus having i6 square feet of area on the foraminous illter elements and operating at a filtration rate of 64 gallons per minute, with a pump capable of supplying such rate against a head of 50 pounds per square inch, a flow restricting passage one-half to threequarter inch long and having a diameter of onequarter inch is lust right. A good size is one that will provide a free cross sectional area of about 0.002 to 0.004 square inch for each square foot of surface of the foraminous filter element, or a free area of 0.0005 to 0.001 square inch for each gallon per minute of pump capacity when operating against a head of about 50 pounds per square inch. The length of the passage should then equal two to three times its diameter. If the passage is made much longer, its diameter must be increased somewhat to retain the same resistance to flow.

If the liquid being iiltered contains relatively large amounts of turbidity it may be advantageous. in the interest of longer illter runs, to continuously add filter aid to the liquid being ltered, termed body feed, in a manner well understood in the art.

While we have disclosed what we consider the preferred apparatus and method in accordance with our invention, modications may be made without departing from its spirit, and reference is. therefore, made to the appended claims for a definition of the scope of our invention.

What we claim is:

1. In a filtering apparatus of the type comprising, in combination, means providing an influent chamber, means providing an eiliuent chamber, foraminous means separating said chambers.

' means for introducing filter aid and liquid to be filtered into said influent chamber, means for conducting iiltered liquid from said eiiiuent chamber. means in both said chambers for trapping in each a predetermined volume of air under pressure and means communicating with said influent chamber for rapidly venting compressed air therefrom to effect backwashing of said foraminous means, the improvement which comprises a closable air vent communicating with the middle portion oi said eiiiuent chamber. and

a flow restricting passage in said air vent so dimensioned as to provide a resistance to ow of air equal t0 that of a passage having a length equal to two or three times its diameter, and a diameter providing a free cross sectional area of 0.002 to 0.004 square inch for each square foot of surface of said foraminous means.

2. In a filtering apparatus of the type comprising, in combination, means providing an influent chamber, means providing Van eluent chamber, foraminousl means separating said chambers, means for introducing lter aid into said inuent chamber, a pump for`introducing liquid to be ltered into said influent chamber, means for conducting filtered liquid from said effluent chamber, means in both said chambers for trapping in each a predetermined volume of air under pressure and means communicating with said influent chamber for rapidly venting compressed air therefrom to eiect baclewashing of said foraminous means, the improvement which comprises a closable air ventcommunicating with the middle portion of said eilluent chamber, and a flow restricting passage in said air vent so dimensioned as to provide a resistance to tlow of air equal to that of a passage having a length equal to two to three times its diameter, and a diameter providing a free cross sectional area of 0.0005 to 0.001 square inch for each gallon per minute capacity of said pump against a head of 50 pounds per square inch.

3. In the operation of a filtering apparatus having a lower influent chamber and an upper eliluent chamber separated from each other by foraminous means, the method of backwashing said foraminous means which comprises draining both said chambers of liquid and lling them with atmospheric air, admitting liquid under pressure into said influent chamber until said air is compressed to a predetermined pressure between 10 and 100 pounds per square inch, exhausting some of said compressed air from said efliuent chamber at a restricted rate of flow and without appreciably lowering the pressure therein while continuing the admission of liquid under pressure until a predetermined volume of compressed air remains in the upper portion of said influent chamber, another predetermined volume of compressed air remains in the upper portion of said eiiluent chamber and the remaining portions of both said chambers are lled with liquid, discontinuing the admission of liquid to said influent chamber and the exhaust of air from said eiuent chamber, and then rapidly venting the trapped volume of compressed air in said inuent chamber to apoint of atmospheric pressure whereupon the volume of compressed air trapped in said eilluent chamber expands rapidly and forces the liquid in said eliiuent chamber at a high rate of flow through said foraminous means.

NORMAN COOPERSON. ALBERT W. KINGSBURY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

